Beam pumping units are widely used in the petroleum industry in order to recover fluids (such as oil) from wells extending into subterranean formations. When the natural pressure of the subterranean oil field is not high enough to force the oil to the surface through the well, a downhole pump must be lowered into the well in order to pump the oil to the surface. Beam pumping units are employed to reciprocate a sucker rod string which extends into the well to actuate the downhole pump. Alternatively, the downhole pump may be actuated by a progressive cavity (or Archimedes' screw) pump which employs rotary rather than reciprocating action.
Referring to FIG. 1, there is illustrated a prior art beam pumping unit, indicated generally at 10. A wellhead 12 of a well which extends from the earth's surface 14 into a subterranean oil producing formation (not shown) comprises the upper portions of a casing string 16 and tubing string 18. The tubing string 18 extends from the wellhead to a suitable depth within the well, e.g. adjacent the subterranean formation. Oil from the well is produced through the tubing string 18 by means of a downhole pump (not shown) to the surface where it passes into a flowline 20.
The downhole pump is actuated by reciprocal movement of a sucker rod string 22. Sucker rod string 22 comprises a polished rod section 22a which extends through a stuffing box (see FIG. 2) of the tubing string 18, and a section 22b formed of a flexible cable. The cable section 22b is connected to a walking beam 24 by means of a "horsehead" 26. Walking beam 24 is pivotally mounted on a sampson post 28 by a pin connection 30. The walking beam 24 is reciprocated by a prime mover 32 such as an electric motor. The prime mover 32 drives the walking beam 24 through a drive system which includes a belt drive 34, crank 36, crank arm 38, and a pitman 40 which is pivotally connected between the crank arm 38 and walking beam 24 by means of pin connections 42 and 44. The outer end of crank arm 38 is provided with a counterweight 46 which balances a portion of the load on the sucker rod string 22a in order to provide for a fairly consistent load on the prime mover 32. It will be recognized by those skilled in the art that the well structure and pumping equipment thus far described are conventional and merely exemplary, and that other suitable pumping units may be utilized in carrying out the present invention.
Referring now to FIG. 2, there is illustrated a prior art wellhead tubing string 18. Threaded connector 50 forms the lower portion of a blowout preventer 52 and is threaded to the end of the tubing string protruding from the wellhead. Blowout preventer 52 contains horizontally opposed sliding rams 54 (only one of which is visible in FIG. 2) which may be moved to sealing engagement with polished rod 22a for the purpose of confining pressure during emergencies or when it is necessary to shut the well in for servicing of the well. Rams 54 may be moved by hydraulic pressure applied at hydraulic valves 56 or by manual screw cranks (not shown) as is commonly known in the art.
A production tee 58 is connected to blowout preventer 52 by means of threaded connection 60. Production tee 58 includes a threaded connector 62 for connection to a flowline 20 (not shown). When oil is pumped to the surface by means of the downhole pump, it flows through blowout preventer 52 (when the rams 54 are in the open position), into production tee 58 and out of threaded connector 62 into the flowline 20.
A stuffing box 64 is connected to production tee 58 by means of threaded connection 66. Stuffing box 64 contains packing material 68 placed in engagement with polished rod 22a in order to form a seal therearound, even as the polished rod 22a reciprocates up and down while working the downhole pump. A cap 71 seals off the top of stuffing box 64 by means of threaded connection 72. Because of the seal provided around polished rod 22a by the packing material 68, no oil may flow above the stuffing box 64. All of the oil pumped to the surface is therefore forced out of production tee 58 and into flowline 20 where it may be collected for storage or transportation, such as by pipeline or truck.
Finally, a polished rod lubricator 70 is placed in frictional engagement with the polished rod 22a at a location immediately above the stuffing box 64. Lubricator 70 rests on top of stuffing box 64 by its own weight. Lubricator 70 contains two reservoirs 74 for holding a quantity of oil. The oil in reservoirs 74 is applied to the polished rod 22a for lubrication thereof by means of oil pads 76 and oil wicks 78. A dust seal 80 is provided for wiping dust and grit off of the polished rod 22a during its downstroke.
The prior art wellhead tubing string arrangement described hereinabove has several drawbacks. First, several feet of height are added to the tubing string above the surface of the wellhead in order to accommodate all of the devices incorporated into the wellhead tubing string. This can be a significant problem in many applications, especially when there is limited available working height above the wellhead due to an overhead obstruction, such as an overhead railroad track used to service the pumps. Additionally, the many threaded connections between the various individual components require seals that are prone to leakage of oil, which has serious environmental consequences and is being increasingly policed by regulatory authorities. Accordingly, a wellhead tubing string which overcomes any or all of these problems is highly desirable. The present invention is directed toward meeting these needs.